1. Field of the Invention
This invention relates to a wheel slip control system for motor vehicles, and more particularly it pertains to a system for controlling the throttle valve of the motor vehicle when the speed change gear is actuated.
2. Description of the Prior Art
There have heretofore been proposed a so-called traction control system for preventing driving wheel slip which tends to occur when the motor vehicle starts running or accelerating while running on a road surface having a low friction coefficient such as, for example, frozen, snow-covered or muddy road surface, thereby improving the starting/accelerating performance and running stability of the motor vehicle, as disclosed, for example, in U.S. Pat. No. 4,722,411 and U.S. Pat. No. 4,736,814.
Referring to FIG. 1 of the drawings, there is shown a timing chart useful for explaining the operation of such a conventional system as referred to just above on the basis of the relationship between driving wheel speed Vd and throttle opening degree command, wherein traction control is effected from time P.sub.1 when the driving wheel speed Vd exceeds a throttle control starting threshold Vs which is set up by adding a predetermined amount to non-driving wheel speed Vn. More specifically, at the time P.sub.1, throttle closing command is provided to drive a DC motor so that the throttle is closed with a predetermined closing rate and engine output is reduced accordingly. At time P.sub.2, the driving wheel speed Vd is changed from increasing to decreasing as a result of the reduction of the engine output, and thereupon throttle opening command is provided to drive the DC motor so that the throttle is opened with a predetermined rate and the engine output is increased accordingly. In this way, it is possible to prevent driving wheel slip which tends to occur when the motor vehicle starts running or acceleration, by controlling the throttle opening degree. At time P.sub.3, shift-up operation of manual transmission is started so that the throttle opening command is cancelled and thus the throttle opening degree command which was prevailing before the gear changing operation, is retained as it is; and at time P.sub.4, throttle opening command is provided again so that the throttle opening degree is increased from that prevailing prior to the gear changing operation.
However, the above-mentioned conventional traction control system is disadvantageous in that there is a tendency that during the period from the time P.sub.1 to P.sub.2, the throttle is excessively closed so that necessary driving force cannot be produced. Another disadvantage is such that during the shift-up operation, the throttle opening degree command provided at the start of the shift-up operation is retained and the actual throttle opening degree is caused to follow that throttle opening degree command to rapidly increase the engine speed so that a significant driving force is suddenly imparted to the driving wheels to the latter to slip at the end of the shift-up operation (when the clutch is engaged).